In yarn brakes of this general type, it is necessary to separate the braking body and the braking surface during threading of the yarn therethrough or during the passage of an enlarged portion of the yarn, e.g. a knot, therebetween. Previously known constructions of yarn brakes of this general type have suffered from problems with respect to the separation of the braking body and the braking surface for these purposes.
One example of a known yarn brake of the general type noted above is described in German Patent Specification No. 15 10 833. In the yarn brake of this patent, a permanent magnet is arranged in a fixed position in the region of the fixed braking surface of the brake housing and this magnet draws a conical braking body consisting of ferromagnetic material against the braking surface of the brake housing by magnetic attraction. For a yarn to be threaded through the yarn passageway, and in particular for it to be so threaded pneumatically, it is necessary for the braking body to be lifted away from the fixed braking surface temporarily so that the yarn can be fed through between the braking surface and the braking body, which are then spaced apart from one another. This lifting or releasing of the movable braking body from the fixed braking surface may be achieved either mechanically or pneumatically, for example by means of a suction flow of air which also serves to thread the yarn through the yarn passageway while at the same time pulling the movable braking body away from the fixed braking surface overcoming the attracting force of the fixed permanent magnet.
However, when the movable braking body is held in its applied position against the fixed braking surface by forces of magnetic attraction, a tendency to "stick" may be observed which is particularly pronounced when the distance between the fixed permanent magnet and the movable ferromagnetic braking body is relatively small. This tendency to stick is a particular disadvantage or nuisance when the yarn is to be threaded through the yarn passageway pneumatically, and thus when the movable braking body is also to be lifted away from the fixed braking surface pneumatically. This undesirable sticking tendency also may occur during the operation of the braking mechanism since any bulge, e.g. a knot, in the yarn must open the brake to allow passage of the bulge therethrough.
Another yarn brake of this general type, as disclosed in U.S. Pat. No. 3,352,510, has attempted to use magnetic repelling force for applying the braking force to the yarn. While avoiding the aforementioned "sticking" problem, this yarn brake has presented additional problems, as will be described. The yarn brake disclosed in this U.S. Patent has a substantially tubular brake housing and a capsule shaped braking body which is situated between a fixed annular braking surface at the top and a movable annular braking surface at the bottom. The movable annular braking surface is mounted so as to be slidably movable in the axial direction, so that the upper and lower annular braking surfaces can be moved into engagement with opposite ends of the capsule shaped braking body. The movable annular braking surface at the bottom is connected to a permanent magnet, with which there is associated in spaced relation a second annular magnet which can be locked at various heights. The two magnets are so arranged that their like poles are situated opposite one another, and the repelling force of the magnets biases the movable annular braking surface upwardly into an operative position wherein the upper and lower braking surfaces are in engagement with the capsule shaped braking body. The sliding mounting for the lower braking surface accommodates for variations in thickness in the yarn running through the yarn brake and also allows for release of the brake during pneumatic threading of the yarn through the brake housing. However, due to the dust and lint which is produced when textile material is being handled, there is a danger of fouling which may interfere with movement of the lower braking surface, with the result that the braking body will no longer be capable of resiliently yielding in response to variations in thickness of the yarn passing through the brake or in response to the pneumatic threading of the filament through the brake housing.